1. Field of the Invention
The present invention relates to low fuming, less hygroscopic resorcinolic novolak resins which are used with curing agents in rubber to give improved mechanical properties and tire cord to rubber adhesion.
2. Brief Description of the Prior Art
Resorcinol and resorcinol-formaldehyde resins have been used in the rubber industry as rubber compounds and adhesives. These resorcinolic compounds and resins are unique materials for rubber compounding since they act as thermosetting or vulcanizing plasticizers. They are very efficient plasticizers for rubber during the processing operations. This allows easier processing, higher loading and excellent extrusions for the rubber compounds.
The thermosetting properties of the resorcinol and resorcinol based resins on curing allow the cured rubber materials to show increase in hardness, abrasion resistance, aging resistance, solvent and oil resistance and stiffness, and also give much improved finishes to the cured rubber stock. This combination of plasticizing and reinforcing action is rare for a single material. In addition, these resorcinolic resins may act as antioxidants when used in the natural rubber.
Although resorcinol-formaldehyde based resins provide good mechanical and adhesion properties, the high fuming and hygroscopicity of these resins present problems in rubber compounding and handling. To solve this problem, tire manufacturers are seeking modified resorcinolic derivatives and resins that do not produce volatiles such as resorcinol at Banbury temperatures. The hygroscopic behavior of resorcinol-formaldehyde resins can be reduced by introducing a hydrophobic aliphatic hydrocarbon chain in the resin molecule. This can be achieved by making use of an alkyl substituted phenol, resorcinol or an aldehyde with a long aliphatic chain in the resin preparation. In the case of an alkyl substituted phenol, this can be reacted with an aldehyde alone or in combination with resorcinol to develop resins using acidic or basic catalysts. In addition to low volatility and hygroscopicity, the new resorcinolic resins should Lave reactivity similar to resorcinol and be non-blooming in the rubber compound.
When the alkylphenols are used with resorcinol, they should be reacted first with aldehydes before adding resorcinol into the reaction mixture because of higher reactivity of the resorcinol as compared to alkylphenols. This procedure will help in reducing the amount of unreacted alkylphenols in the resulting resin.
The prior art on such resin development is well described in U.S. Pat. No. 4,167,540. This patent discloses that nonylphenol modified resorcinolic resin is made by reacting first nonylphenol and paraformaldehyde (91% active) using toluene sulfonic acid as the catalyst and then resorcinol is added to produce a novalak type resin.
U.S. Pat. No. 4,257,926 describes the parasubstituted phenol modified resorcinol-formaldehyde resins using alkaline catalysts. This patent discloses that the resoles are produced first by reacting p-alkylphenols with paraformaldehyde or acetaldehyde using sodium hydroxide as the catalyst. Then the formed resoles, without isolation, are allowed to condense with resorcinol to produce the novolak type resins. In these resin preparations, 88% paraformaldehyde is used and toluene is used as the azeotroping solvent.
United Kingdom Patent 1,564,235. discloses a heat reactive water-based adhesive composition comprising a water soluble, or water dispersible novolak phenolic resin, a methylene donor and water.
U.S. Pat. No. 4,257,926 discloses a method of preparing a co-polycondensate resin by condensing (A) a resol type pre-polycondensate prepared by at least one phenol and formaldehyde or acetaldehyde in the presence of an alkaline catalyst, and (B) resorcinol, m-aminophenol or the like while distilling off water present in the reaction system, and then solidifying the reaction mixture.
In spite of these prior art disclosures, there remains a very real and substantial need for an effective resorcinolic novolak resin which will provide a wide variety of desirable properties and that may be used with curing agents to give improved mechanical properties to rubber and improved tire cord to rubber adhesion.
It is, therefore, an object of the present invention to provide a resorcinolic novolak resin, containing furfural, cashew nut shell liquid (CARDOLITE NC-511) or an allylphenol, that is low fuming, less hygroscopic compared to resorcinol formaldehyde resin and capable of cross-linking with curing agents in the rubber to give improved mechanical properties and improved tire cord to rubber adhesion properties.